For the ST1100, when you purchase the European decals (the ones that say PanEuropean, rather than ST1100), the decals come affixed to a piece of backing paper that provides a template for correctly locating the decal. If you take a close look at the backing paper, you will see that there is a compound curve cut into the backing paper that exactly matches the compound curve at the aft end of the front fairing (see pictures below). So, to install the decal, you first pre-position it (do a test fit, so to speak) with the backing paper still affixed, so that you know where the decal is supposed to go and you understand the correct positioning of it, then, you remove the film that protects the adhesive side of the decal (whilst leaving the film on the other side that provides the positioning guidance) and stick the decal on.
After you have firmly affixed it, smoothed out the bubbles, etc., you peel the positioning template off the front surface and throw it away.
Michael
What the decal looks like when you get it, and how to position it